1. Field of the Invention
The present invention relates to laser scanning systems for reading bar code symbols, and more particularly, to motor control circuits for hand held portable laser scanning heads.
2. Description of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail business, has resulted in the development of various bar code reading systems. Many users of bar code readers require portable hand held scanners which place a premium of size, weight and power requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831, assigned to the same assignee as the present invention and incorporated by reference herein.
The laser scanning system disclosed in U.S. Pat. No., 4,496,831 includes a portable hand held scanning head which may be embodied in various shapes but preferably has a gunshaped housing made of lightweight but flexible plastic. A handle and barrel portion are provided to house the various components of the scanning head therein. Within the barrel portion are mounted a miniature light source, a miniature optic train including focusing lenses for directing the light source, miniature scanning means for sweeping the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned. The handle portion generally supports a DC power supply.
The miniature light source comprises a laser tube such as a co-axial helium-neon laser tube, or preferably, a semiconductor laser diode, which is considerably smaller and lighter than a laser tube, thus reducing the required size and weight of the scanning head, and making the scanning head easier to handle and more maneuverable. Light generated by the light source passes through the optic train which focuses the beam to impinge upon the scanning means, which are mounted in the light path within the barrel portion of the scanning head. The scanning means sweeps the laser beam across the bar code symbol, and comprises at least one scanning motor for sweeping the beam lengthwise across the symbol, and preferably comprises two motors, where the second motor sweeps the beam widthwise across the symbol. Light reflecting means such as mirrors are mounted on the motor shafts to direct the beam through the outlet port to the symbol. The sensing means then detects and processes the light reflected off the symbol, and generally comprises photosensitive elements such as semiconductor photodiodes.
The structural aspects of the scanning motor are analogous to a stepper motor which is a device used to convert electrical pulses into discrete mechanical angular movements every time the polarity of a stator winding in changed. When this structure is used as a scanning motor, a scanning motor control means is employed to oscillate the shaft first in one circumferential direction over an arc length less than 360.degree., and secondly in the opposite circumferential direction over an arc length less than 360.degree., and thereafter to repeat the aforementioned cycle at a high rate of speed.
The motor control means for the above prior art scanning head includes a reference means for applying a generally constant low level direct current voltage to one of the stator windings, thereby energizing one set of stator poles as north and south. The motor control means also includes a variable means for applying a periodic voltage of time-varying amplitude to the second set of stator windings, thereby energizing the second set of stator poles as north and south. By varying the polarity and frequency of the periodic voltage, the rotor will align itself accordingly. The high speed scanning motor is controlled in such a way as to cause the motor shaft to move in an analog manner to an angular position which corresponds to the amplitude of an applied periodic voltage, with the number of degrees of angular movement being proportional to the periodic voltage amplitude. The speed of movement of the shaft corresponds to the rate of change of the periodic voltage, and the direction of movement is dependent upon the slope of the periodic voltage, whose waveform may be any time-varying voltage, such as a sine wave, triangular wave, sawtooth wave, or the like. Moreover, it is recognized that by adjusting the voltage applied to the direct current stator windings to the resonant frequency of the motor, the frequency of movement will be at or near resonance resulting in a reduction of the required power. However, in the control circuit of the above referenced patent, two power supplies are required, one of which continuously supplies a constant low level direct current voltage to one of the stator windings.
In another aspect of the above referenced patent, a closed loop control circuit is disclosed to provide linear tracking of the laser scanning beam. The control circuit comprises a primary coil, two or more secondary coils and a moveable shield. The shield is fixedly mounted on the motor shaft for joint oscillatory movement therewith, and is located between the primary coil and two secondary coils. Tuning capacitors are used to tune the secondary coils to resonate at the frequency at which the primary coil is excited, the secondary coils are inductively coupled to it to establish an oscillating magnetic field, and an AC voltage of the same high oscillating frequency as the primary coil appears across the secondary coils, and is detected by sensing circuitry. The sensing circuitry detects the voltage on its respectively associated secondary coil, and feeds the voltages to a differential amplifier for generating a difference signal which is proportional to the angular displacement of the shaft. This difference signal is, in turn, fed to one input of another differential amplifier whose input is supplied with a control voltage. The output of the amplifier is then fed to the stator coil. The aforementioned patent, therefore, sets forth an elaborate system to control the amplitude of oscillation of the motor shaft. Although the control circuit of the above referenced patent provides a means for monitoring and adjusting the amplitude of oscillation of the motor shaft, the present invention allows the amplitude to be adjusted or set initially and does not require continuous monitoring or regulation.